The present invention relates to a filter press type electrolyzer, and in particular, to an electrolyzer characterized by circulation of electrolytic solution.
The electrolyzer of filter press type is widely used in various applications such as manufacture of chlorine and caustic soda by electrolysis of salt, or electrolytic manufacture of organic substances, electrolysis of seawater, etc.
In a typical electrolysis method using the filter press type electrolyzer for electrolysis of salt, a bipolar type filter press type electrolyzer is used, in which a plurality of electrolyzer units are placed one upon another via cation exchange membrane, and anode chamber and cathode chamber adjacent to each other are connected electrically and mechanically via partition walls in the electrolyzer units. On both ends, end type electrode chamber units each having anode or cathode on each side thereof are placed on each other, and these are fixed by hydraulic press or other means.
On the other hand, in this bipolar type electrolyzer unit, partition walls are provided to separate the anode chamber from cathode chamber and also to transmit electric current for electrolysis. On the partition wall to separate anode chamber from cathode chamber, anode and cathode are mounted respectively. Depending upon each individual electrolysis reaction, one of the anode chamber and the cathode chamber is in acidic environment, while the other is in reducing environment. In particular, in the electrolysis of salt, i.e. typical electrolysis method utilizing ion exchange membrane, chlorine is generated at anode, and high concentration sodium hydroxide and hydrogen are generated at cathode. In this respect, thin film forming metal such as titanium, tantalum, zirconium, etc. with high corrosion-resistant property, resistant to chlorine, or alloy of these metals are used in the anode chamber. In the atmosphere of the cathode chamber, titanium absorbs hydrogen and is embrittled, and even highly corrosion resistant titanium cannot be used for the cathode chamber. For this reason, ferrous metal such as nickel, stainless steel, etc. or alloy of these metals are used for the cathode chamber. By forming each of these electrode chambers by partition walls made of metal materials and by connecting these chambers together, electrical connection can be achieved. However, if it is tried to connect titanium on the anode chamber side directly with iron, nickel, stainless steel, etc. on the cathode side by welding, intermetallic compound is formed by titanium and ferrous metal on the anode chamber side, and it is not possible to obtain a bonded system, which has sufficient strength suitable for practical application.
To solve these problems, the present applicant filed JP-A-03249189, disclosing a bipolar electrolyzer, which comprises partition walls with irregular surfaces engaged with each other and produced by press procedure, and a structure of electrolyzer units with electrode connected on a convex portion and a method to manufacture the electrolyzer units. Further, the present applicant proposed an electrolyzer with improvement in circulation of electrolytic solution within the bipolar electrolyzer in JP 5005195A (U.S. Pat. No. 5,314,591), JP 5005196A (U.S. Pat. No. 5,314,591), or JP 5009774A (U.S. Pat. No. 5,314,591), etc.
In particular, by the method proposed in JP 5009774A (U.S. Pat. No. 5,314,591), it is possible to achieve better electrical connection through the irregular surfaces on the partition walls. By improving circulation of electrolytic solution in the electrolyzer, even distribution of concentration of the electrolytic solution can be attained, and efficient operation of electrolyzer can be realized.
In the electrolyzers of this type, a system for circulation of electrolytic solution in the electrolyzer is adopted with the purpose of supplying electrolytic solution evenly over the extensive electrode area.
FIG. 6 is a drawing to explain a method to circulate the electrolytic solution by external circulation of electrolytic solution.
From an electrolytic solution inlet 18 on the lower portion of an electrolyzer unit 1, electrolytic solution 31 is introduced into an electrode chamber 4, and the electrolytic solution containing electrolysis products is discharged from a discharge port 32 on the upper portion of the electrolyzer and this is collected in a circulation tank 33. In the circulation tank 33, gas products 34 are separated, and a part of the discharged electrolytic solution is sent to an electrolytic solution preparation process 35, and at least a part of the electrolytic solution in the circulation tank 33 is mixed with a supplementary or make-up solution 36, and this is supplied through the electrolytic solution inlet 18 on the lower portion of the electrolyzer into the electrolyzer using a circulation pump 37, and the solution is circulated.
In case the electrolytic solution is brine or salt water, brine with concentration of 200 g/l is mixed with saturated brine with concentration of 300 g/l at volume ratio of 1:1, and if it is supplied as brine with concentration of 250 g/l, difference in the concentration of the electrolytic solution between the electrolytic solution inlet 18 and the discharge port 32 is 50 g/l.
In order to reduce the concentration difference of the electrolytic solution between the inlet and the discharge port, there is a method to increase the circulation volume of the electrolytic solution and to circulate a larger quantity of electrolytic solution. However, when flow rate is increased, pressure fluctuation in the upper portion of the electrode chamber is increased, and ion exchange membrane dividing anode chamber from cathode chamber is vibrated, and this leads to deterioration of the ion exchange membrane.
Further, FIG. 7 is a schematical drawing to explain a method to circulate electrolytic solution, utilizing the difference in specific gravity of the electrolytic solution caused by electrolysis.
An electrolytic solution tank 38 is provided, which is connected to a discharge port 32 of the electrolyzer in the upper portion of an electrolyzer unit 1, and a pipe on the lower portion of the electrolytic solution tank is connected to an electrolytic solution inlet 18. Electrolysis products containing gases generated in the electrolyzer are moved upward in the electrolyzer because of the difference in specific gravity and reach the electrolytic solution tank 38. In the electrolytic solution tank 38, gas products 34 are separated, and a part of the electrolytic solution is sent to electrolytic solution preparation process 35, and a supplementary solution 36 is added to a part of the electrolytic solution to adjust concentration of the electrolytic solution, and this solution is supplied from the electrolytic solution inlet 18 into the electrode chamber 4.
When the electrolytic solution is supplied to the lower portion of the electrolyzer equipped with an electrolytic solution circulation system as described above, the electrolytic solution is diluted. The concentration of the electrolytic solution at a position away from the electrolytic solution inlet cannot be evenly distributed. Thus, distribution of electric current becomes uneven near the electrolytic solution inlet of the electrode chamber, and this adversely affects voltage for electrolysis.
In case brine is electrolyzed, hydrochloric acid is often added to the brine in order to reduce pH value of the electrolytic solution. Because of uneven distribution of concentration in the electrolytic solution, lower pH occurs near the electrolytic solution inlet, and this often leads to deterioration of ion exchange membrane.
It is an object of the present invention to prevent uneven distribution of concentration and temperature in the electrolytic solution in electrode chambers, to improve voltage and current efficiency and to provide longer service life of ion exchange membrane. In particular, the invention provides an electrolyzer, by which sufficiently high electrolysis performance can be attained in a large size electrolyzer with larger electrode area.